In a laser gyro, two monochromatic beams of light are generated and caused to travel in opposite directions about a closed loop path perpendicular to the axis about which rotation is to be sensed. As the gyro is rotated, the effective path length for one beam is increased while the effective path length for the other beam is decreased. Because the frequency of oscillation of a laser is dependent upon the length of the lasing path, a frequency difference between the two beams is produced. The magnitude and sign of this frequency difference is indicative of the rate and direction of rotation of the gyro about its sensitive axis and may be monitored to provide the desired gyro output.
As the rate of rotation being sensed is decreased, the frequency difference between the two beams is similarly decreased. At very low rates of rotation, errors arise due to "lock-in" effects, whereby no frequency difference between the beams is observed. Lock-in arises where the frequency splitting between the two beams is small, causing coupling from one beam into the other beam so that the beams oscillate at the same frequency. This results in a dead band or lock-in region wherein the gyro output does not track the input.
Various dither techniques have been employed in an attempt to eliminate lock-in at low rates of rotation. One such technique is to provide a dither motor to vibrate the body of the gyro with a sinusoidal dither at the natural frequency of the assembly as shown in Killpatrick Pat. No. 3,373,650. Another technique employed to minimize the effects of lock-in at low rates of rotation is shown in Killpatrick Pat. No. 3,467,472 which provides the addition of a random noise input to the sinusoidal drive signal for a gyro-dither motor assembly such as shown in Killpatrick Pat. No. 3,373,650. Each of the gyro-dither motor assemblies shown in the Killpatrick patents is a high Q assembly so that it may be driven at its natural frequency.
A rigorous analysis of the effects of a sinusoidal body dither and a sinusoidal body dither having a random noise input for a laser gyro as shown in the Killpatrick patents, is given in the article entitled "Scale Factor Nonlinearity of a Body Dither Laser Gyro" by Thomas J. Hutchings and Daryl C. Stjern (1978). This article shows that with a sinusoidal body dither, there are significant scale factor errors or nonlinearities in the output of the gyro, not only at very low input rates, but also at input rates which are a harmonic of the sinusoidal dither frequency. The article also shows that with the addition of a random noise input signal to the sinusoidal body dither, the scale factor errors or nonlinearities which result when the input rate is a harmonic of the dither frequency are somewhat reduced, but not eliminated. The body dithered laser gyro used to illustrate these effects, like all known prior art body dithered laser gyros, was an assembly having a high Q.
Known body dithered laser gyros have specifically been made to have a relatively high Q, i.e., a Q of 100 or more, because very little power is required to drive the dither motor supporting the gyro when it is driven at the natural frequency of the assembly. However, the effect of such a high Q assembly is that while a random noise input signal may be added to the sinusoidal drive for the dither motor, the assembly responds only to those noise frequency components of the drive signal which are at or very close to the natural frequency of the assembly. As a result, the body dither actually obtained with such a high Q assembly appears to be a sinusoidal dither with a small amount of amplitude modulation as a result of the frequency components in the random noise input which are close to the natural frequency of the assembly. Noise frequency components which are much different in frequency will have a negligible effect on the actual dither achieved, there being no frequency modulation of the dither motion. The small amount of amplitude modulation of the sinusoidal dither may result in a reduction in the width of the nonlinearities caused when the input rate of the gyro is a harmonic of the dither frequency, but does not completely eliminate the nonlinearities.
Further, with a high Q gyro-dither motor assembly, the amplitude of vibration imparted to the gyro is affected by other factors such as external vibratory inputs. Thus, such high Q assemblies require the use of feedback circuits to drive the gyro-dither motor at the natural frequency of the assembly in order to maintain the desired amplitude. Where laser gyros are employed in an instrument cluster, each of the gyros must have its own feedback circuit to which its associated dither motor is responsive.
In an instrument cluster having three gyro-dither motor assemblies each being positioned on a different one of the X, Y and Z axes, it has also been found that when the gyros are driven at the same frequency, they tend to beat together. This causes the instrument cluster to move in a manner which defines a cone, this motion resulting in so-called coning errors. In order to prevent coning errors from arising, each of the gyros forming the instrument cluster must be driven at a different frequency. Where the instrument cluster is comprised of three high Q gyro-dither motor assemblies, the structure of each assembly must be made different, so that each assembly has a different natural frequency at which it is driven. The high Q gyro-dither motor assemblies of such an instrument cluster are therefore not interchangeable. That is, an X-axis assembly cannot be used to replace a Y-axis assembly. This results in the need to maintain a large inventory of the three different types of gyro-dither motor assemblies.